Purpose : To assess the incidence and causes of pseudohydronephrosis in the abdominal two-phase (arterial andportal) spiral CT. Materials and Methods : This study included 115 renal pelvises of 62 patients (hepatocellularcarcinoma in 36, gastric carcinoma in 10 and others in 16 patients) whose entire renal pelvises were imaged inboth phases of abdominal spiral CT. Arterial and portal phase scans were obtained 30 and 65-70 seconds after theinjection of 100 mL of non-ionic contrast media (370mg I/mL) at a rate of 2.5 mL/sec, respectively. The renalpelvis was considered to be dilated when the shortest diameter of renal pelvis exceeded 1cm as seen on CT. In eachpatient, a plain film was obtained 5-10 minutes after the injection of contrast media for CT to confirm thehydronephrosis. Results : Seven renal pelvises were dilated on spiral CT. All of them were pseudohydronephrosiswith prominent renal pelvis and two of them were extrarenal pelvises. In 115 renal pelvises, there was one case oftrue hydronephrosis due to a ureteral stone not suspected clinically. Conclusion : A 6% incidence ofpseudohydronephrosis due to prominent renal pelvis should be considered in interpreting the abdominal two-phasespiral CT.